A conventional indirectly heated cathode comprises a hot cathode emitting electrons at one end and a tubular sleeve comprising a molybdenum material and having a heater inside. In order to enhance the capacity of the tubular sleeve to absorb radiant heat from the heater, an aluminum layer is coated on the inside face of the tubular sleeve and heat-treated in wet hydrogen to form a blackened thermal absorption layer on the inside face of the tubular sleeve (Publication of Japanese Patent Application (Tokkai Sho) No. 56-169778).
However, there are some disadvantages in such an indirectly heated cathode where Al.sub.3 Mo is formed on the thermal absorption layer. One of the disadvantages is that while such an indirectly heated cathode integrated into a cathode ray tube is operated, Al.sub.3 Mo evaporates from the thermal absorption layer of the indirectly heated cathode. The vapor of Al.sub.3 Mo may attach to an electrode located in the vicinity of an indirectly heated cathode, thus causing insulation breakdown. Evaporation of Al.sub.3 Mo may make the black thermal absorption layer less black, preventing the tubular sleeve from efficiently absorbing radiant heat from the heater.